> The two missing libraries are part of the package itself; apparently, they
> do not get encoded into the rpath. Could this be handled somehow by the build
> system? Should it be done in the package definition? Or should I set an
> LD_LIBRARY_PATH?

You can adjust RPATH with patchelf.  Here’s an example [1] (ignore the
comment about shared libs).

Does it answer your question?

[1] https://lists.gnu.org/archive/html/guix-devel/2013-07/txt5Qs_mNF2lH.txt